Processes exist in the industry which use those aromatic polyamines, in particular M-DEA and M-DMA, as curing agents for certain epoxy resins such as the various types of ARALDITE.RTM., EPIKOTE.RTM. resins, EPON.RTM. resins, the resin obtained from the diglycidylether of bis-phenol-F, abbreviated here to DGEBF, and in particular the resin obtained from the diglycidylether of bis-phenol-A, abbreviated to DGEBA. In such processes, prior dissolution of the M-DEA or M-DMA in the DGEBA resin before impregnation into the reinforcing fibre, for example glass fibre, carbon fibre, ceramic fibre or a natural or synthetic fibre, is expressly recommended. The polyamine is normally dissolved in the epoxy resin to form a homogeneous solution using two principal methods, which are summarised below.
In a first method, the aromatic polyamine is dissolved in the epoxy resin which has been preheated to a temperature which is higher than the melting point of the polyamine used, or higher than the melting point of the polyamine with the highest melting point when a mixture of polyamines is used. Dissolution generally takes at least 15 minutes, very often 15 to 30 minutes. During that phase, the temperature must be kept at least at the melting point of the polyamine with the highest melting point in order to obtain a homogeneous solution. If necessary, the dissolution period is increased until a homogeneous solution is obtained. Despite rapid cooling, the fact that the polyamine is dissolved when hot, i.e., at a temperature which is at least equal to its melting point, causes the reaction (polyaddition) to begin, thus requiring the user to store the resin or prepreg at low temperatures, i.e., at a temperature which is sufficiently low for the polyaddition reaction to be severely limited and to thus obtain at a usable life of several tens of days, or several months. Without cold storage, i.e., at a minimum of minus 18.degree. C., the usable life of the homogeneous mixture obtained would only be a few hours, which is completely unsatisfactory for the envisaged use of such compositions.
The second recommended method consists of dissolving the polyamine or mixture of polyamines at low temperature in a suitable solvent with a relatively low boiling point, less than 100.degree. C. for example. The solvent is, for example, a ketone such as methyl-ethyl-ketone, abbreviated here to MEK. The polyamine solution is then added to the epoxy resin. This mixing method, which avoids heating to a high temperature, nevertheless requires subsequent elimination of the solvent.
There is a particular problem in this case, however, with solvent elimination, which problem is linked to the particular polarity of the polyamine-epoxy resin mixture and requires drying by heating to a temperature of 100.degree. C. for several minutes, particular when MEK is used as the solvent. Despite these precautions, about 0.8% by weight of solvent remains in the prepreg. This residual solvent limits the use of such a process to thin laminates, as the solvent bubbles when hot pressing (the pressing temperature is usually above about 120.degree. C.) causes undesirable porosity which can be an impediment to the envisaged use of the prepreg. Such prepregs can only be used for thick composites (over 1 millimeter thick, for example, usually less than 10 centimeters and generally about 5 millimeters to about 5 centimeters thick) if a vacuum contraction cavity coupled with a pressurized autoclave is used, thus severely complicating implementation of such a method. Further, it has been noted that the usable life of such a prepreg is usually of the order of several days and less than one week at ambient temperature and is only a few hours, generally less than 5 hours, at 80.degree. C., which is completely unsatisfactory for the envisaged use.
Clearly, such processes cannot be applied when drying is carried out under mild conditions, i.e., at a moderate temperature, for example less than about 100.degree. C.
In a third method, the polyamine or mixture of polyamines is mechanically ground into very fine particles, normally less than about 50 microns (1 micron=10.sup.-6 meter) then introduced into the epoxy resin, or the epoxy resin and the polyamine or mixture of polyamines are ground together in an apparatus such as a three-cylinder mill, etc. Such mechanical methods cannot produce homogeneous mixtures simply and safely.
Prior grinding of the polyamine or mixture of polyamines to a very fine powder requires the use of a rigorous protection protocol against micronized aromatic amine dust since such substances are relatively toxic and the user must be protected and must avoid all contact with and inhalation of such dust.
Grinding a polyamine-epoxy resin mixture requires carrying out at least the three following successive steps:
a) mixing a polyamine or polyamine mixture type curing agent with the epoxy resin, normally requiring sufficient heat to cause the dynamic viscosity of the medium to fall in order to be able to form as homogeneous a mixture as possible; PA1 b) passing the mixture obtained in step (a) into a grinding apparatus, for example a three-cylinder mill. During this grinding step, the polyamine particles and the epoxy resin particles segregate on the rollers. This phenomenon is limited when the polyamine-epoxy resin mixture is passed through the grinding system several times; PA1 c) finally, the product obtained in step (b), which is normally not sufficiently homogeneous, is sent to an apparatus which can produce a homogeneous mixture. This apparatus can, for example, be an anchor reactor. Such a method normally results in large losses of product and involves a large number of manipulations. PA1 it must be capable of producing a pot life or usable life if supported on reinforcing fibers, for example glass fibre or the equivalent thereof, which is greater than or equal to 20 days, normally about 30 days to about 4 months, usually about 30 days to about 2 months, for an average storage temperature which is less than or equal to 25.degree. C., normally about 0.degree. C. to about 15.degree. C., and usually about 0.degree. C. to about 5.degree. C.; PA1 it must be capable of producing a residual latency of at least 6 hours after pot storage or supported storage under the conditions described above, preferably at least 8 hours, at a temperature of about 70.degree. C. to about 90.degree. C., usually about 80.degree. C.; PA1 it must be capable of producing a residual latency of at least 8 hours, preferably at least 10 hours, and usually at least 11 hours, at a temperature of about 70.degree. C. to about 90.degree. C., usually about 80.degree. C., after pot storage or supported storage for a period of at least 90 days, normally about 90 days to about 6 months, and usually about 90 days to about 4 months, at a temperature of less than or equal to minus 18.degree. C., normally about -30.degree. C. to about -18.degree. C., and usually about -25.degree. C. to about -18.degree. C.; PA1 it must be capable of being deposited in conventional fashion, manually or mechanically, on a fibrous support such as glass fibre, carbon fiber, KEVLAR.RTM. or a metal fibre or wire, on ceramic fibre or on natural or synthetic fibre,; and PA1 finally, it must satisfy health and safety regulations.
Even at moderate temperatures, the time required to produce such a suspension of polyamine in the epoxy resin by mechanical means is sufficiently long to have a substantial and deleterious influence on the latency period which becomes much lower than the desired period for the envisaged use.
These methods, which are known to the skilled person, are thus not applicable to the production of a prepolymer which must satisfy the following conditions: